1. COS 260 DAY 19
Tony Gauvin

2. Agenda Questions? 8th Mini Quiz today Assignment 4 Corrected
Good results
many “similar” assignments
I know that the answer keys are posted online, it doesn’t mean that you can use them
Assignment 5 Posted
Due November 19
2nd Capstone progress reports due
Timing of proposal and reports is 10% of project grade
Discussion on More on Inheritance

3. More about inheritance
Objects First with Java
More about inheritance
Exploring polymorphism
5.0
© David J. Barnes and Michael Kölling

4. Main concepts to be covered
method polymorphism
static and dynamic type
overriding
dynamic method lookup
protected access
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

5. The inheritance hierarchy
Project network-v2
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

6. Conflicting output What we want What we have Leonardo da Vinci
Had a great idea this morning.
But now I forgot what it was. Something to do with flying ...
40 seconds ago - 2 people like this.
No comments.
Alexander Graham Bell
[experiment.jpg]
I think I might call this thing 'telephone'.
12 minutes ago people like this.
What we want
Leonardo da Vinci
40 seconds ago people like this.
No comments.
Alexander Graham Bell
12 minutes ago people like this.
What we have
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

7. The problem The display method in Post only prints the common fields.
Inheritance is a one-way street:
A subclass inherits the superclass fields.
The superclass knows nothing about its subclass’s fields.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

8. Attempting to solve the problem Ex 9.1
Place display where it has access to the information it needs.
Each subclass has its own version.
But Post’s fields are private.
NewsFeed cannot find a display method in Post.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

9. Static type and dynamic type
A more complex type hierarchy requires further concepts to describe it.
Some new terminology:
static type
dynamic type
method dispatch/lookup
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

10. Static and dynamic type
What is the type of c1?
Car c1 = new Car();
What is the type of v1?
Vehicle v1 = new Car();
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

11. Static and dynamic type
The declared type of a variable is its static type.
The type of the object a variable refers to is its dynamic type.
The compiler’s job is to check for static-type violations. for(Post post : posts) { post.display(); // Compile-time error. }
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

12. Overriding: the solution
display method in both super- and subclasses.
Satisfies both static and dynamic type checking.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

13. Overriding
Superclass and subclass define methods with the same signature.
Each has access to the fields of its class.
Superclass satisfies static type check.
Subclass method is called at runtime – it overrides the superclass version.
What becomes of the superclass version?
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

14. Distinct static and dynamic types
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

15. Method lookup No inheritance or polymorphism.
The obvious method is selected.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

16. Method lookup
Inheritance but no overriding. The inheritance hierarchy is ascended, searching for a match.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

17. Method lookup
Polymorphism and overriding. The ‘first’ version found is used.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

18. Method lookup summary The variable is accessed.
The object stored in the variable is found.
The class of the object is found.
The class is searched for a method match.
If no match is found, the superclass is searched.
This is repeated until a match is found, or the class hierarchy is exhausted.
Overriding methods take precedence.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

19. Super call in methods Overridden methods are hidden ...
... but we often still want to be able to call them.
An overridden method can be called from the method that overrides it.
super.method(...)
Compare with the use of super in constructors.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

20. Calling an overridden method
public void display() {
super.display();
System.out.println(" [" +
filename +
"]");
System.out.println(" " + caption); }
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

21. Method polymorphism
We have been discussing polymorphic method dispatch.
A polymorphic variable can store objects of varying types.
Method calls are polymorphic.
The actual method called depends on the dynamic object type.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

22. The instanceof operator
Used to determine the dynamic type.
Recovers ‘lost’ type information.
Usually precedes assignment with a cast to the dynamic type: if(post instanceof MessagePost) { MessagePost msg = (MessagePost) post; … access MessagePost methods via msg … }
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

23. The Object class’s methods
Methods in Object are inherited by all classes.
Any of these may be overridden.
The toString method is commonly overridden:
public String toString()
Returns a string representation of the object.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

24. Overriding toString in Post
public String toString() {
String text = username + "\n" +
timeString(timestamp);
if(likes > 0) {
text += " - " + likes + " people like this.\n"; }
else {
text += "\n";
if(comments.isEmpty()) {
return text + " No comments.\n";
return text + " " + comments.size() +
" comment(s). Click here to view.\n";
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

25. Overriding toString
Explicit print methods can often be omitted from a class:
System.out.println(post.toString());
Calls to println with just an object automatically result in toString being called: System.out.println(post);
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

26. StringBuilder
Consider using StringBuilder as an alternative to concatenation: StringBuilder builder = new StringBuilder(); builder.append(username); builder.append('\n'); builder.append(timeString(timestamp)); … return builder.toString();
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

27. Object equality What does it mean for two objects to be ‘the same’?
Reference equality.
Content equality.
Compare the use of == with equals() between strings.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

28. Overriding equals public boolean equals(Object obj) {
if(this == obj) {
return true; }
if(!(obj instanceof ThisType)) {
return false; }
ThisType other = (ThisType) obj;
… compare fields of this and other }
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

29. Overriding equals in Student
public boolean equals(Object obj) {
if(this == obj) {
return true; }
if(!(obj instanceof Student)) {
return false; }
Student other = (Student) obj;
return name.equals(other.name) &&
id.equals(other.id) &&
credits == other.credits;
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

30. Overriding hashCode in Student
/**
* Hashcode technique taken from
* Effective Java by Joshua Bloch. */
public int hashCode() {
int result = 17;
result = 37 * result + name.hashCode();
result = 37 * result + id.hashCode();
result = 37 * result + credits;
return result; }
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

31. Protected access
Private access in the superclass may be too restrictive for a subclass.
The closer inheritance relationship is supported by protected access.
Protected access is more restricted than public access.
We still recommend keeping fields private.
Define protected accessors and mutators.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

32. Access levels
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

33. Review The declared type of a variable is its static type.
Compilers check static types.
The type of an object is its dynamic type.
Dynamic types are used at runtime.
Methods may be overridden in a subclass.
Method lookup starts with the dynamic type.
Protected access supports inheritance.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling